The present invention is directed to a microwave oven excitation system and more specifically to a system which couples a continuously moving asymetric field into the oven cavity so as to provide improved uniformity of field distribution within the oven cavity during a selected cooking cycle.
In a microwave oven cooking cavity, the spatial distribution of the microwave energy tends to be non-uniform. As a result, "hot spots" and "cold spots" are produced at different locations. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed. The problem becomes more severe with foods of low thermal conductivity which do not readily conduct heat from the areas which are heated by the microwave energy to those areas which are not. An example of a food falling within this class is cake. However, other foods frequently cooked in microwave ovens, such as meat, also produce unsatisfactory cooking results if the distribution of microwave energy within the oven cavity is not uniform.
One explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes", are set up within the cooking cavity. When a standing wave pattern is established, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern is dependent upon a number of factors including the frequency of microwave energy used to excite the cavity and the dimensions of the cavity itself. It is possible theoretically to predict the particular mode patterns which may be present in the cavity, but actual experimental results are not always consistent with theory. This is particularly so in microwave ovens operating at a frequency of 2450 MHz. Due to the relatively large number of theoretically possible modes, it is difficult to predict with certainty which of the modes will exist. The situation is further complicated by the differing loading effects of different types and quantities of food which may be placed in the cooking cavity.
A number of different approaches to altering the standing wave patterns have been tried in an effort to alleviate the problem of non-uniform energy distribution. The most common approach is the use of a device known as a "mode stirrer", which typically resembles a fan having metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself or within a recess formed in one of the cooking cavity walls, typically located at the top of the oven cavity. The intended function of the mode stirrer is to alter the mode pattern in the oven cavity so that the "hot" and "cold" spots are continually shifted resulting in the energy distribution in the cavity being made more uniform when averaged over a period of time.
Although the use of a mode stirrer improves energy distribution in the cavity, it has been found in practice that uneven energy distribution can still exist. For example, depending on the characteristics of a particular cavity and the feed aperture used to inject the microwave energy into the cavity, it is possible to have a region at one side of the cavity at a significantly higher strength than exists on the opposite side. Uneven distribution can also occur in the front to back direction.
Another approach to achieving more uniform cooking of food load in the oven is to employ a rotating table on which the food load is placed. The theory is that as the food load is rotated through "hot" and "cold" spots in the mode pattern, the averaged heating of the food will result in relatively uniform cooking. Although somewhat helpful to this end, in actual practice, the results depend on the particular mode pattern established in a given oven and on the nature of the food load. For example, a vertically polarized predominantly TE mode will not perform satisfactorily in cooking horizontally placed bacon strips despite the use of the rotating table. Moreover, a mode pattern that produces a low energy level in the center of the oven will cause the axial portion of the rotating food load to remain less well cooked than in the peripheral sections of the load which pass through higher energy regions in the cavity.
Yet another prior art approach involves the use of a rotating antenna within the oven cavity intended to couple a rotating horizontal field into the cavity. However, it has been found that this approach is not entirely satisfactory apparently due to the fact that the vertical field coupled into the cavity is the predominant mode and is, of course, stationary. The minor horizontal field coupled by the rotating horizontal antenna segment is inadequate to significantly alter the more dominant stationary vertical field modes and, as a result, field uniformity in the oven cavity is not desirably realized.
It is, therefore, an object of the present invention to provide an excitation system for a microwave oven that will serve to improve the uniformity of energy distribution in the oven cavity during a cooking cycle over presently known techniques.